In the drilling of oil and gas wells and in oil and gas production facilities, flare stacks and/or blooie lines are used, through which combustible gases, off-gassed from the wellbore, are released and burned. The release of gas through the flare stack or blooie line is typically intermittent and has non-predictable rates, including low velocity flow, creating the potential for backflash, which is the advancing of the flame front back through the flow to the source of the gas.
During the drilling of oil and gas wells, using a variety of drilling fluids including, but not limited to air, mist, foam, aerated and liquid mud systems, the release of combustible gases is most likely to occur while drilling at balanced or underbalanced phases of well control. Air drilling operations, whether straight air, mist or foam, are particularly at risk for backflash and, particularly so, when stopping and starting the flow of air to the wellbore while making and breaking drillpipe connections. After connection and following commencement of the flow of air in the drillpipe, it takes some time before the air completes the circuit downhole and back to surface, thus leaving a lower gas velocity below the flare igniter and therefore creating the potential for backflash.
Generally, backflash is most likely to occur where there is a combination of three factors, namely; a low to zero velocity flow of a combustible air and hydrocarbon gas mixture through the flare stack or blooie line; the combustible gas mixture is contained in a finite structure within the flare stack and/or blooie line or other structure; and there is a means for igniting the combustible gas mixture. One such typical example exists in a flare stack line extending from a separator vessel or a blooie line extending from the wellhead in underbalanced or balanced drilling wherein a combustible gas mixture flows from the wellbore flow tee, diverter or rotating diverter head or the separator to the flare stack and/or blooie line having an outlet to the atmosphere, the flare stack and/or blooie line being equipped with a continuous ignition source.
As described in “Flammability and Flashback Prevention (a work in progress)” by Dan Banks, P. E posted on the worldwide web at www.banksengineering.com/about_flame_arrestors_and_detona.htm, flame progresses at a defined rate through a combustible mixture. If the flow velocity of the gas mixture through the flare stack and/or blooie line falls below a minimum gas velocity, the minimum gas velocity being a velocity greater than a flame propagation velocity, the flame is capable of moving upstream from the point of ignition to the source of the gas and igniting the gas therein. For example, in the case of a methane/air mixture, the velocity in the pipe must exceed 1.5 ft/sec to prevent flame propagation upstream to the ignition source. If the gas source of the combustible mixture is at the separator, the separator is at risk of explosion; or if the flame front of the backflash travels down into the wellbore, a downhole fire and possibly an explosion is likely, which could result in the loss of the entire well section.
Typically, conventional underbalanced separators utilize backpressure valves during balanced and underbalanced drilling operations to attempt to prevent backflash however, in some circumstances the backflash can still occur through the backpressure valve. Further, pressure maintained in the separator as a result of the backpressure valve retards entrained gas from evolving from the drilling fluids in the separator. As drilling fluids are passed to a shale shaker, entrained gas which did not evolve in the separator can evolve at the shaker, creating a fire potential or the potential for the release of carcinogenic and toxic gases. The backpressure valve may also result in the exertion of a higher bottom hole pressure on the formation which can interfere with underbalanced drilling. In the case of blooie line systems, it is typical that no backflash systems are employed. In either case, it is known in the industry that backflashes to separator vessels and into wellbores have occurred, resulting in compromise to the structural integrity of mud/gas separators and causing underground fires. In Canada, backflashes have been experienced by a number of companies, particularly while air hammer drilling and/or foam drilling.
As reported by Susan Eaton in New Technology Magazine, March 2002 “Conquering Foothills Challenges—the air force”, air drilling can be dangerous, risky and costly, and underground fires are a real danger. As suggested, successes have been realized using a combination of air and nitrogen or nitrogen alone to replace combustible mixtures with air, however providing a source of compressed nitrogen suitable for use in the volumes required for air drilling is costly and requires additional specialized equipment at surface.
In cases where a large influx of fluids or gas, called a “kick”, is encountered or predicted while drilling, the operator typically shuts the blowout preventer (BOP), weights up the drilling fluid and commences drilling again using a heavier drilling fluid to increase the hydrostatic head in the wellbore which is capable of suppressing or minimizing the fluid influx. Cessation of drilling and weighting up the drilling fluid results in lost drilling time and decreased rates of penetration (ROP).
Clearly what is needed is a simple, reliable system for handling drilling fluids, particularly where “kicks” may be anticipated, that permits the evolution of gases from the drilling fluids within a separator for eliminating evolution of gas at the shale shaker preventing backflash, uncontrolled release of gas at the shaker tank or fear of environmental contamination. Further, it is desirable that the system permit continued drilling despite the intermittent influx of combustible hydrocarbons so as to maintain high ROP's.